The present invention relates to a thermal conductivity gage with a probe and a separate operating unit, the latter containing an indicator and a power supply for a temperature-dependent resistance element in the probe.
Thermal conductivity vacuum (pressure) gages make use of the fact that, at higher gas pressures, i.e., greater molecule densities, more heat is removed by the gas from a temperature-dependent resistance element in thermal communication with the gas than at lower gas pressures. In the Pirani thermal conductivity vacuum gage, the resistance element is a measuring wire which is connected in a Wheatstone bridge. In the unregulated type of Pirani vacuum gage, a change in the resistance of the measuring wire produces an imbalance in the bridge which is used as a measure of the pressure. In the regulated type of Pirani vacuum gage, the voltage applied across the bridge is constantly regulated in such a way that the resistance and hence the temperature of the measuring wire is constant. This means that the bridge is always balanced. In formerly known regulated Pirani vacuum gages, the voltage across the bridge is then used as the measure of the gas pressure.
In thermal conductivity vacuum gages which operate on the thermoelectric principle, one or more thermocouples are provided as the temperature-dependent resistance element through which a supply current flows. Such thermal conductivity vacuum gages can also be unregulated or regulated.
In all these thermal conductivity vacuum gages there is the disadvantage that small voltage variations must be transmitted from the probe to the operating unit. This makes gage accuracy dependent upon the resistance of the conductors. Furthermore, the transmission of low measuring voltages is unreliable, especially when the point at which the pressure is to be sensed (the probe) is relatively far from the operating unit powering and indicating the sensed pressure.